Concrete resistivity for grounding grid

[lume7006]

Hello everybody,

We are doing a design verification of a grounding grid and in the surface layer, there is only concrete.

Could anybody provide us the value of concrete "air crude" for dry or wet?

At IEEE std 80 it is stated a value like 1 x 10 6 Ohms-m, however, it is also mentioned that this value is only for an specific kind of concrete.

Any informaion will be welcomed!

Thank you in advance

 

[jghrist]

The IEEE-80 value of 1 x 10^6 ohm-m is for oven-dried concrete, not normal construction concrete. A better figure would be the 30-90 ohm-m given in Section 14.6, Concrete-encased electrodes.

If the concrete is reinforced, you should not be considering it as an insulating layer like crushed rock. It will be more like an equipotential surface. You could model it in detail as a separate soil volume if you know the rebar locations. What I have done is to just model a conservative (minimum) amount of rebar buried in the soil but not connected to the ground grid. If the concrete has lower resistivity than the soil, the calculation of touch- and step-voltage will be conservative using the soil resistivity. In this situation, I would not consider any insulating layer.

 

[ppaya]

Jghrist:
is pavement (ashfalt) considered insulator?

 

[jghrist]

Yes, asphalt pavement would be an effective insulating layer, but not reinforced concrete pavement.

 

[cuky2000]

Insulator may be defined as a material that cannot conduct electricity under normal conditions. However, a rupture of insulation that results in a substantial transient or steady increase in leakage current at the specified voltage.

For grounding applications, the asphalt may have high resistivity but cannot be considered as an insulator. For practical purposes the surfacing material could be considered as a resistance added to the body/shoes resistance model resulting in an increase of the allowable step and touch potentials.

Concrete may be considered as a semiconducting material. The electrical resistivity of concrete depends upon the capillary pore size, pore system complexity and moisture content. Resistivity is strongly influenced by concrete quality: cement content, water/cement ratio, admixtures and curing regime, among other factors.
CONDITIONS\MATERIAL CONCRETE ASPHALT
Outdoor applications (wet): low (20-100 Ohm.m) High resistivity (~10,000 Ohm-m)
Indoor application high (10,000 Ohm-m) Very high